Gravity, magnetic and structural patterns at the deep-crustal plate boundary zone between West- and East-Gondwana in Sri Lanka

Precambrian Research, 66 (1994) 77-91

77

Elsevier Science B.V., Amsterdam

Gravity, magnetic and structural patterns at the deep-crustal plate boundary zone between West- and East-Gondwana in Sri Lanka G. Biichel Institut J~r Geowissenschaften, Universiti~t Mainz, Postfach 3980, 55 099 Mainz, Germany Received March 23, 1992; revised version accepted August 26, 1992

ABSTRACT Detailed gravity and magnetic field measurements were carried out in 1989 and 1990 to investigate the eastern boundary between the late Archaean granulite complex of the Highland (HC) and the Neoproterozoic amphibolite-grade complex of the Vijayan (VC) along four profiles. Based on the density determination of 320 rock samples, the average density of HC rocks is 2.78 g/cm 3, and of VC rocks 2.65 g/cm 3. All 2D models of the residual Bouguer gravity anomalies reveal the HC rocks to be plate wedges with a thickness of about 1-3 km overlying the VC rocks. Within the border zone between HC and VC a new rock unit with an average density of 2.80 g/cm 3 could be defined mainly by geological, less by geophysical data. This rock unit consists of migmatized former charnockitic hornblende-biotite-garnet-gneisses and is called here "Mahiyangana rocks" and "Wellawaya rocks", respectively, after the local villages. From the viewpoint of the isotopes, they should be VC rocks, but in fact they represent an independent rock wedge between HC and VC. Field observations show that these three rock units are separated by shear zones. The interpretation of lineation and foliation data indicates that the rock piles were overthrusted in a N-S direction like nappes similar to ramp and flat tectonics. Later, a so-called F 5 folding phase caused the curved trend of the former N-S striking boundaries. The HC, assumed to be a passive continental" margin of West-Gondwana, collided in the lower crustal realm with the VC and the "Mahiyangana rocks", and "Wellawaya rocks", a metavolcanic sequence of an active continental margin probably as part of East-Gondwana, during the PanAfrican event.

1. Geological background Sri Lanka mostly consists of Precambrian lower crustal crystalline rocks. Upper Jurassic sediments are preserved within three small fault-bounded basins in the west of the country. Along the northwest-coast and in the extreme north (Jaffna-peninsula) Miocene sediments are exposed (Fig. 1 ). From east to west the basement of Sri Lanka can be divided in simplified terms into Vijayan Complex (VC) (East-Vijayan), High-

land Complex (HC) (Highland~Southwestern Complex or Central Granulite Belt) and Wanni Complex ( W C ) (West-Vijayan) (Cooray, 1984, 1994; K r r n e r et al., 1991; Voll

and Kleinschrodt, 1991 b). The predominant Highland Complex extends as a belt of granulite facies rocks from the south coast to the Central Highlands up to the north-northwestcoast at Trincomalee. In the southeast, several islands of HC rocks occur within VC rocks. The best known enclave is located near Kataragama. A significant part of the HC rocks are metasediments, probably more than half of them are of plutonic origin. The Vijayan Complex, which comprises the east and the southeast of Sri Lanka, consists of granitoid tonalities to leukogranites of amphibolite-facies. Contrary to many orthogneisses of the HC, the orthogneisses of the VC do not originate from anatexis of older crustal mate-

0301-9268/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0 3 0 1 - 9 2 6 8 ( 9 3 ) E 0 0 2 7 - A

sion exists. Furthermore, the HC rocks are approximately 1 Ga older than the VC rocks. The clear-cut differences, compiled in Table 1, were the cause for a geophysical investigation of this deep-crustal plate boundary.

821

ca

2. Geophysical surveys

i I !

~MA

I Quaternary

~

I

I Vijayan Complex (VC)

Miocene

~

Wanni Complex (WC)

Upper Jurassic ~

Highland Complex (HC) PRE - CAMBRIAN

Fig. 1. Simplified geological map ofSri Lanka after Cooray (1984) with the locations of the four investigated areas: Mahiyangana, Uraniya, Wellawaya and Kataragama. Note that the boundary between HC and VC is not well defined.

rial, but represent differentiates of a mantle magma (Milisenda, 1991 ). The mignaatized rocks of the Wanni complex are not described in this paper. In both rock units, HC and VC, a distinct foliation (s2) is evident. Depending on the rock type, it is characterized by flattened quartz, biotite, hornblende, feldspar, sillimanite and pyroxene. In most cases the lithological banding runs parallel to the foliation. At the boundary between HC and VC the foliation gently dips to the west or northwest. For this reason, Hatherton et al. (1975) and later Vitanage ( 1985 ) and Kr6ner (1986) assumed that the more highly m e t a m o r p h o s e d HC rocks rest upon the less metamorphosed VC rocks, which means that an inverse metamorphic sucees-

Detailed gravity and some magnetic field measurements were carried out along four profiles perpendicular to the postulated boundary between HC and VC (Figs. 1, 2, and 3). The distances between field stations varied between tens of metres and a little more than 1 km, depending on the field conditions. Hatherton et al. ( 1975 ) carried out a gravity reconnaissance survey of Sri Lanka, based on 1000 data points at bench marks, but the resulting smooth gravity map does not resolve the boundary between HC and VC in detail. Along our new profiles, which are between 25 and 38 km long, only two to three gravity points of Hatherton et al. are located, whereas we measured between 50 and 110 gravity points along each profile. We could not use bench marks due to new buildings and uncertain locations. Therefore, the data points were levelled forwards and backwards with an infrared laser instrument (ET-2, Topcon). For the gravity measurements a Worden gravimeter (No. 238) was used; control measurements were carried out with a Sodin gravimeter (Geodetic 100 GT). Coordinates and altitudes were calculated with the help of a levelling computer program with reference to control points of the Sri Lanka Geodetic Survey. These are mostly trigonometric points from the topographic map 1:63,360; the coordinates were obtained more or less completely from the local surveying departments. The program indicates potential errors (e.g. reading errors) larger than the standard deviation (Biichel et al., in prep. ). All gravimetric reductions were calculated with a standard rock density of 2.67 g / c m 3. For the terrain reduction a three-dimensional top-

GRAVITY,MAGNETICANDSTRUCTURALPATTERN,SRILANKA TABLE 1 Differences of the Highland and the Vijayan Complex, modified after Kr/Sner ( 1991b ) and Voll and Kleinschrodt ( 1991a) Highland Complex

Vijayan Complex

Lithologies Metasediments: garnet-sillimanite-gneisses(khondalites), quartzites and quartzitic gneisses, marbles and calc-silicate rocks, quartz-feldspar-granulites Granitoid anatectic gneisses: charnockites, quartz-feldspar-granulites

Juvenile orthogneisses (tonalites, leukogranites ): granitic domes, migmatites (biotit-hornblende-gneisses), Rarely: small inclusions of quartzites, calc-silicate rocks and marbles

Geochemical pattern Alkaline

Calc-alkaline

Geotectonic environments Passive continental margin (shelf)

Active continental margin (?)

Depositional ages Ages of detritical zircons in metapelites: 1.9 to 2.3 Ga (maximum depositional age) in orthogneisses: 1.85 to 1.9 Ga (minimum depositional age)

Ages of detritical zircons in quartzites: 1.1 Ga (maximum depositional age) in orthogneisses: 1.02 Ga (minimum depositional age)

Nd-model ages 2.2-3 Ga

1-2 Ga

Grades of metamorphism Predominantly granulite-grade

amphibolite-grade

Palaeopressure and palaeotemperatare 8-9 kbar, 700-800°C

6 kbar, 700°C

P - T path

Isobaric cooling

Isobaric cooling?

Migmatitization Rarely

Mostly before 0.58 Ga

Ages of metamorphism Between 0.67 and 0.55 Ga

Between 0.58 and 0,51 Ga

Topographies Upland surface 150-750 m highland surface 1500 to 1800 m

Lowland surface 30-120 m

Gravity anomalies Positive Bouguer anomalies up to 10 mGal

Negative Bouguer anomalies

Folding phases Fl to F4

Fl, F4

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Fig. 2. Location maps of the four geophysical profiles, which follow the roads (bold lines).

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GRAVITY. MAGNETIC AND STRUCTURAL PATTERN, SRI LANKA

81

W

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Fig. 3. Bouguer gravity anomalies along four profiles and magnetic total intensity on two profiles crossing the boundary between HC and VC. For the location of the profiles see Fig. 2. The assumed contact between HC and VC, taken from the geological map of Sri Lanka (Geol. Surv. Dept. of Sri Lanka, 1982 ), is drawn in ordinary capitals. Newly defined contacts between HC, MR, WR and VC are drawn in bold (see text).

ographic model (TTM) was computed on the basis o f the topographic map 1:63 360, using a 200 m × 200 m grid close to the profile and a 400 m × 400 m grid further away. The TTM is adapted to the altitudes o f the levelling points: up to a distance o f 240 m between data points the TTM is corrected to the surveyed altitude; from 240 to 400 m the correction decreases linearly from 100% to 0%. To calculate the ter-

rain reduction, the TTM is divided into blocks and the gravity effects are added up for all blocks up to a radius of about 3000 m. If the distance between the centre of a block and a measurement point is smaller than the grid width, the blocks close to the point are subdivided into 25 smaller blocks, whose altitudes are computed with a bicubic spline-interpolation using the surrounding 24 TTM-points. The

standard deviation of the terrain reduction does not exceed _+0.5 mGal, including the standard deviation of the levelling and the instrument variations. In Fig. 3, the Bouguer anomalies along the real length of the measured profile are plotted. A regional trend is not taken into account as later for the modelling in Fig. 7. In each profile the lowest gravity value is set to 0 mGal. The magnetic data were obtained at the same locations as the gravity stations. The presented data in Fig. 3 result from the respective average values of several measurements of total magnetic intensity around gravity stations.

3. Interpretation The results of the gravity and magnetic survey are plotted in four profiles (Fig. 3). The topography obtained by levelling is added. The Mahiyangana profile can be subdivided into two parts: The western part of the profile is characterized by a positive Bouguer anomaly, which begins abruptly at km 14 just west of the Mahaweli Ganga and ascends to the west to a relative value of + 18 mGal. The eastern part of the Mahiyangana profile shows a significantly smoother curve. The values vary between + 5 and 0 mGal. Only near the east end of the profile at km 32 a higher gradient appears. It is located just west of the Navaragala hill. From the geological field data the western part of the profile correlates with HC rocks. Typically they appear as interbedded garnetsillimanite-gneisses, marbles, calc-silicate rocks and quartzites. Garnet-sillimanite-gneisses with a very variable content of garnet predominate. This interbedded rock complex can be mapped from the west end of the profile to km 12 in the east. In one of the last outcrops 2 km before the geophysical boundary, south of the tank northeast of Hasalaka, intensively stretched garnet-siUimanite-gneiss was identified. Near the geophysical boundary at km 14 no outcrops are present.

East of the Mahaweli Ganga from km t7.5 to km 31 migmatized hornblende-biotite-garnet-gneisses occur, which are strongly banded and sometimes are developed as light coarsegrained feldspar-rich and sometimes as dark hornblende-biotite-rich layers. Extremely rare small fragments (metre-scale) of marbles and/ or calc-silicate rocks occur within this homogeneous rock sequence. Possibly these are tectonically interbedded wedges. Additionally, massive charnockites predominate north and northeast of Mahiyangana, but they are not noticeable in the gravity profile. Presumably the hornblende-biotite-garnet-gneisses east of Mahiyangana are migmatized former charnockitic rocks, retrogressively changed to amphibolite-facies. Just east of these charnockites, at km 26 of the profile, the assumed contact between HC and VC in the geological map of Sri Lanka (Geological Survey Department, 1982) is located. Considering the former granulite-facies metamorphism, these hornblende-biotitegarnet-gneisses east of Mahiyangana should be HC rocks, but from the viewpoint of the isotopes (Milisenda et al., 1988; Liew et al., 1991 ) they should be VC rocks. Due to their specific characteristics the term "Mahiyangana rocks" (MR) is introduced here which includes the "transitional zone" of Cooray ( 1969 ). Typical amphibolite-facies light biotitehornblende-gneisses without garnet begin around km 31, west of the late tectonic gneiss dome of Navaragala, which is cut by the profile in its northern part. The gneiss dome is characterized by a small positive Bouguer anomaly of about + 2 mGal. Surprisingly, the boundary between MR and VC is not noticeable in the geophysical profile very well, although the density contrast between MR and VC is - 0 . 1 4 g / c m 3 (s.b.). This might be due to the gravity effect of the Navaragala gneiss dome. Outside the profile further to the east, the VC rocks envelope numerous gneiss domes like that of Navaragala. The Uraniya profile is located southeast of

GRAVITY, MAGNETIC AND STRUCTURAL PATTERN, SRI LANKA

83

the Mahiyangana profile (Fig. 2 ). It follows the small roads full of bends. In the NNE-SSWrunning part of the profile a rapid ascent of + 3 mGal to the south-southwest is obvious. In the eastern part of the profile a negative anomaly of - 3 mGal appears. It correlates with the topographic crest. East of this negative anomaly the gravity curve ascends to the east corresponding to the regional trend (e.g. Fig. 6 ). From the geological point of view the typical HC rock assemblage, which is only badly exposed, can be m a p p e d from the west end of the profile to the western border of the topographic crest. Additionally, clinopyroxenehornblende-garnet-gneisses occur. East of the topographic crest typical VC rocks crop out. They contain small intercalations of marbles. One of these is exposed close to the village of Petiyagoda. These are "intrusive" marbles. About 1.5 km east of these marbles the boundary between HC and VC rocks is assumed in the geological map of Sri Lanka. The topographic crest is caused by a pink granitic gneiss dome which consists of microcline, quartz, and hornblende. Biotite, present in very small contents, traces the foliation. The gneiss dome is characterized by a negative Bouguer anomaly in contrast to the Navaragala gneiss dome of the Mahiyangana profile, which is located in its north-northwest extension. The boundary between HC and VC in the geophysical profile is not very significant. In agreement with the geological field data the boundary is located just west of the granitic gneiss dome. Here, the gravity curve begins to ascend to the west. In the northern corner of the profile around Anda Ulpota relatively low gravity values strike the eye. It is assumed that here, between km 12 and 16, "Mahiyangana rocks" are present. The gravity curve ascending to the west is very similar to the ascent observed at the boundary between HC and M R in the Mahiyangana profile. Why are M R lacking between HC and VC in the Uraniya profile, in contrast to the Mahi-

yangana profile? The author believes that the MR are overridden by the HC rocks. So, in the Uraniya profile the HC rock assemblage extends further to the east. The contact between HC and MR must be a tectonic contact. The curves of the magnetic anomalies mostly run inverse to the Bouguer anomalies. Low gravity values coincide with positive magnetic anomalies and vice versa. Generally, the magnetic total intensity in the HC rocks is higher than in the VC rocks. The Wellawaya profile is subdivided into two parts. Part I in the north shows a "noisy" gravity curve with anomalies of short wavelengths. The magnetic curve is significantly smoother except for the negative anomaly at km 2.5. Here, heavy charnockitic gneisses are located. In contrast to part I of the profile, part II shows a smooth gravity and a very "noisy" magnetic curve. The high magnetic gradients result from pegmatitic dikes, which often contain massive magnetite. The so geophysically identified new boundary between parts I and II is located at km 10, 2 km north of Wellawaya, where no outcrops exist. However, in the geological map of Sri Lanka this boundary is fixed 2.5 km south of Wellawaya. From the geological field data part I of the profile is characterized by the typical rock assemblage of HC rocks. In contrast, in part II, a homogeneous sequence of hornblende-biotite-garnet-gneisses occurs. Only a small outcrop of garnet-sillimanite-gneisses could be found 9 km south of Wellawaya. Considering the similarities between the rocks south of Wellawaya and the "Mahiyangana rocks", the term "Wellawaya rocks" (WR) analogous to M R is introduced here. At the I / I I boundary between HC and WR, west of Wellawaya, a homogenous shear zone about 50 m thick was discovered, which, according to its coarse crystallinity, experienced a later tempering (Fig. 4). The Kataragama klippe is a well known enclave of HC rocks in the VC. The gravity profile cuts this klippe in N - S direction. In the area

file, where also marbles predominate close to the assumed thrust contact. 4. Densities

Fig. 4. Dark hornblende-biotite-gneisses and lighter granitoid gneisses in a shear zone 1 km west of Wellawaya (south of the road). The coarse-grained sheared rocks indicate crystal growth during high temperature conditions after shearing due to the strong migmatitization. This rock type, about 50 m in thickness, is assumed to represent the boundary between two completely different geotectonic environments.

of HC rocks a small positive "noisy" Bouguer anomaly occurs. With regard to the geological field data the boundary to the VC (?) rocks is located at km 14 in the south and km 3 in the north, corresponding to the boundaries of the positive gravity anomaly. In contrast to these results, the assumed contact between HC and VC in the geological map of Sri Lanka is located at km 17 in the south and km 0 in the north. The Kataragama profile surprises by its abundance of marbles and charnockites. The marbles appear to be the lubricant of the thrust zone, probably similar to the Wellawaya pro-

For the analysis of the gravity results, especially the modelling, the densities of 320 rock samples were determined in the field by weighing them in air and in water with two spring balances (measuring range 0 to 5 N and 0 to 20 N, respectively). For verification the determination of 80 rock densities was repeated in the laboratory. The results of the density measurements are complex (Fig. 5): on the one hand, the HC consists of a distinct interbedding of different rock types with different densities, on the other hand, the densities also vary strongly within areas with only one or two rock types of one rock group. For example, the rock group of garnet-sillimanite-gneisses, predominant in the Highland near Mahiyangana, shows a density range between 2.60 and 3.05 g/cm 3 due to its varying garnet content, For the gravity modelling it is necessary to calculate average densities for the respective rock units, in relation to their mapped area. Surprisingly, the result is consistent in that the average densities of HC rocks in all four profiles fall within the narrow interval of 2.75 to 2.80 g/cm 3. The densities of the light granitoid VC rocks generally are smaller by 0.07 to 0.17 g/cm 3 than the HC rocks. The densities of the biotite-hornblende-garnet-rocks, retrogressively metamorphosed to amphibolite-facies (MR and WR), occurring between HC and VC in the Mahiyangana and Wellawaya profiles, differ only slightly from the HC rocks. 5. Modelling For the modelling, only the residual gravity anomalies were used, as shown later in Fig. 7. The regional trend was derived from the smoothed Bouguer gravity map of Hatherton et al. ( 1975 ). The anomaly of the whole island of Sri Lanka of up to - 100 mGal was defined

are

GRAVITY, MAGNETIC AND STRUCTURAL PATTERN, SRI LANKA

[%1 35 3O

VIJAYAN COMPLEX Rock Densities

2520-15-10-5--

~v.e~g.g~ 2,65 0 2,5

~.~Z-~..

[NI

[g/cm31

I/a.. AI ---q l ~. . . . I , 2,8

2,9

I

3,0

1

3,1

I I°

0 I 3,2 [g/cm3]

Mahiyangana 2.63 g/crn3 ~Uraniya 2.68 g/cm3 20--

MAHIYANGANA ROCKS

Is-

[%1 area ~ ~ ~

10 --

5

~ - - ~ WELLAWAYA ROCKS Rock Densities

~

I 20

IN]

--

2,5

2,6

2,7 ~ - ~

Mahiyangana 2.77 g/cm3

[%1 area 20-

\\

~-.~

2,9

3,0

3,1

10

3,2 Ig/cm3]

Wellawaya2.83 g/cm3

HIGHLAND COMPLEX Rock Densities

I

2,5

I

2,6

I

Iz'3,'N

2,7~ ~z~

i

2,9

I

3,0

I

3,1

I

3,2 [g/cm3]

Wellawava 2.78 g/crn~~ - ' - ~ ~-Mahiyangana 2.80 g/cm3 tJraniya 2.75 g/cm~ Xataragama 2.79 g/cm~

Fig. 5. Rock density histograms of the working areas. The striped histograms represent the densities related to the percentage of the mapped area of the respective rock type in the area surrounding the profiles (% area). The dotted histograms list the densities in relation to the n u m b e r of the rock samples. The average densities refer to the %area-values.

85

as regional and is due to the lower densities of the supracrustal continental crust in relation to the higher densities of the surrounding oceanic crust to the south and east of the island (Curray and Moore, 1971; Kahle et al., 1981; Curray et al., 1982; Liu et al., 1982; Curray, 1991 ). The calculation of the regional trend is demonstrated for the Mahiyangana profile as an example (Fig. 6). For the modelling the average densities of the respective rock units in the profiles were used. The gravity stations of the curved gravity profiles were projected on E - W or N - S profiles. The residual Bouguer anomalies were modelled with simple two-dimensional bodies by trial and error (Fig. 7 ). The design of the model bodies was decided by geological field observations and, above all, average rock densities. With the exception of Wellawaya, the profiles cross the tectonic features (boundary between HC, VC and MR, F4 folds, lithological banding) about perpendicular. So the basic requirement for 2D modelling is fulfilled. The Wellawaya profile crosses the boundary between HC and WR about perpendicular, but the rest of the tectonic features oblique! In this case, only a rough adjustment was modelled (Fig. 7 ). Also in the other three profiles, the gravity anomalies with high gradients were not completely fitted, because the small anomalies often could be attributed to 3D rock bodies. It is assumed, that these are heavy mafic and light pegmatitic rocks. All four forward models reveal the HC rocks to be a plate wedge with different thicknesses. The Mahiyangana and Uraniya profiles show a sharp stepped contact to the MR and VC rocks. The first step in the Mahiyangana profile, at km 22 l, probably correlates with garnet-sillimanite-gneisses, identified 2 km west of the boundary. The second big step at x-coordinate 215 km correlates with a large eastfacing F4 fold in a topographic escarpment of about 700 m. The migmatized hornblendebiotite-garnet-gneisses east of the H C / M R boundary are modelled as a flatly inclined plate wedge. It is assumed that they underlie the HC

c~ E >~

b~=-0.950489-04, b4=O.139309-06

-i

-q

o~ o rn

-50 I l

5O

I

tOO

150 200 250 x-coordinates [km]

300

I

I

350

Fig. 6. Bouguer gravity anomaly along an E-W profile through Sri Lanka crossing Mahiyangana. The squares and stars represent the intersected contour lines of the gravity map by Hatherton et al. (1975 ). Only the points marked by squares were used for the calculation of the regional trend using a polynom equation of the fourth order. The stars indicate the positive Bouguer anomaly of the HC in relation to this regional trend due to this higher density. Note that the maximum regional anomaly is twice as high as the continuation of the gravity profile to the east. Assuming a density contrast of 0.2 g/cm 3 between the lower crustal rocks of Sri Lanka (average density about 2.8 g/cm 3) and theoceanic crust (estimated average density of about 3.0 g/cm 3, including a sediment cover of Bengal Fan of about 3 kin, Curray, 1991 ), the continental crust has a thickness of more than 25 km.

rocks in the western part of the profile. No significant differences in Bouguer anomalies are observed at the boundary between MR and VC rocks. This is possibly caused by the Navaragala gneiss dome and/or by the only gently dipping contact between the VC rocks and the overlying MR (Fig. 7 ). The modelled bodies of the Uraniya profile are similar to those of Mahiyangana. The almost isolated lens of HC rocks in the foreland of the big HC plate wedge correlates with a big F4 east-facing syncline, one of the biggest F4 folds of Sri Lanka. Between the syncline filled by HC rocks and the plate wedge in the west, most likely MR occur. VC granitoid gneiss domes become apparent as small negative (x-coordinates 241 to 243 km within the Uraniya profile) and small positive anomalies (x-coordinates 236 to 238 km within the Mahiyangana profile). The reason for this is unknown.

In the Wellawaya prof'fle the average density of HC rocks is by -0.05 g/cm 3 smaller than the migmatized hornblende-biotite-garnetgneisses of Wellawaya (Wit). A HC plate wedge of only about 1 km thickness could be modelled. The calculated small heavy bodies in the south with a density of 2.93 g/cm 3 consist of hornblende-rich layers in the hornblende-biotite-garnet sequence. Based on an average density contrast of + O.14 g/cm 3, the Kataragama klippe was modeUed as a plate wedge, highly inclined on both sides, with a maximum thickness of 1.2 kin. The northern continuation of the profile indicates a positive residual gravity anomaly of about + 2 regal. No VC rocks seem to be present there due to their high modelled density. 6. S~uemr~ Seoh~y Collision of the three rock complexesis implied by the structural geological data. The re-

-1

calculated

observed

WELLAWAYA

(*I (----I

1 1-1

(W

r

lo--5

-0

km

cn

calculated

observed

Irl (--_)

y-coordinates

KATARAGAMA

(km)

Fig. 7. 2D models of the upper crust to fit the four profiles. In the Mahiyangana profile two flat-lying plates of HC (waves) and MR (dots) were modelled. The model bodies of gneiss domes are indicated by crosses. In the Uraniya profile only the boundary between HC and VC (no signature) was calculated. It is possible that the MR are located at the base of the HC plate wedge. In the Wellawaya profile only the boundary between HC and WR was modelled, in the Kataragama profile the boundary between HC and VC(?).

1

r”lrllln+DA

observed

Mahiyangana •

~

_

=

_

and V C T--° _ f .... / f ...

\

-

7

~. .....

\ < ~ >/

lawa

/z

/

/ Kataragama

657

all tectonic data foliations, 313 lineations

..~...,~

:

I:... •

•

iiii~i

-

.. ~,. ¢f

.

:. .

~: :~ ~'" . . . ~ ~~.:;~.-

..:.'.

Y 110 krr]j

t~--4

"- ....

; ....

Fig. 8. Left side: tectonic data from the four profiles. Bottom: summary of all tectonic data• The poles of foliations (contour lines) define a great circle, which describes the large-scale folds (F4), whereas the maxima of lineation poles (dots) represent the fold axes of the F, folds. Right side: the same Schmidt nets as on the left side, in their topographic location relative to each other. The F4 fold axes curve from N-S to NE-SW due to a F5 younger folding phase after the emplacement of nappes.

GRAVITY, MAGNETIC AND STRUCTURAL PATTERN, SRI LANKA

89

sults of the lineation (longitudinal axes of quartz, hornblende and sillimanite) and foliation measurements are shown in Fig. 8. The foliation is folded twice around the stretching linear (F2, only in the central part of HC, and F4, Voll and Kleinschrodt, 199 lb ); the later F4 phase of folding forms open to isoclinal largescale folds with a wavelength of hundreds of metres to kilometres. The poles of the foliations reveal a significant east-facing of these large-scale folds in the two northern profiles. Towards the south, gradually open upright folds can be observed. The fold axes of these large-scale folds correspond with the maxima of the lineation poles. Within the Mahiyangana profile they strike N-S and show extremely shallow dips. Within the two southern profiles of Uraniya and Wellawaya the strike direction of the fold axes changes from N-S to NW-SE. The fold axes dip increasingly towards the NW. In the Kataragama region the fold axes are turned by 90 ° . The preferred strike direction of foliation and lineation now is NE-SW. The fold axes of the large-scale folds again are horizontal. Near the contact to the VC the F4 folds are locally overprinted by open flexure-glide folds, which also folded the thrust plane slightly (Kleinschrodt, 1994). The direction of the lineations indicate the transport directions of the rock complexes (Fig. 8 ). The rock piles were overthrust in the N-S direction as nappes and accumulated as a pile of nappes. The analysis of south-facing F3 folds within the HC near Nildandahinna (Voll, 1991 ) demonstrates that the upper part of the nappe pile was transported from north to south over the lower nappes. From north to south an undulation of nappes can be interpreted from the dip of the lineations towards NNW and NW in the Uraniya and Wellawaya profiles. Within the Kataragama region the transport is again horizontal, similar to ramp and flat tectonics. The bending of the foliation and lineation from N-S (Mahiyangana) to NNW-SSE (Uraniya) and N W - S E (Wellawaya) and finally to NE-SW (Kataragama) is attributed to

the latest folding phase, which is called F5 here (Fig. 8 ). The collision of the four rock units could have happened (a) during or (b) before the F4 large-scale folding, because the lineation and foliation distribution of HC, MR, WR, and VC is approximately identical within each profile (Fig. 8). In the case of (b), the thrust plane must have been folded as strongly as the F4 folds. This could not be observed in the field. In contrast, from the structural data of the big F4 folds it can be deduced that the contact between the rock units does not run parallel to the folded lithological banding. So the collision of rock piles happened during the formation of the F4 large-scale folding, possibly towards the end of this tectonic event. This means the thrust planes are only weakly folded, but this folding cannot be resolved by gravity modelling. The large-scale folds could have been formed during the N-S transport of the nappe pile along an oblique ramp (R. Kleinschrodt, pers. commun., 1989). Several lineations perpendicular to the directions of large-scale fold axes within the Uraniya and Wellawaya profiles (Fig. 8) indicate an E-W compression, younger than the nappe emplacement. Later, the F5 folding caused the curved trend of the F4 large-scale folds. 7. Conclusions

According to the shape of the Bouguer gravity curves and the geological field data along the profiles, four different rock units are distinguished within the border zone between HC and VC in the eastern part of Sri Lanka. Considering the results of the gravity modelling, MR and HC rocks resemble plate wedges of small thickness. The formerly assumed boundary between HC and VC rocks (drawn in the geological map of Sri Lanka) was newly mapped and defined by these combined geophysical/geological investigations (Figs. 3, 7 ). Two new rock units (MR and WR) were in-

~" J

~I~ ~

"

~

Highland Comp[ex(HC)

Vijayan Comptex (VC) Mahiyangana Rocks (MR} Wel[awaya Rocks (WR) Fig. 9. Schematic block diagram showing the formation of the boundary between HC and VC. This is also the boundary between West-Gondwana (East Africa) and East-Gondwana (East Antarctica).

troduced. The boundaries between HC, VC, MR, and WR are evidently shear zones. These four rock units are derived from completely different geotectonic environments: The metasediments of HC, about 2 Ga old, originated from a passive continental shelf. The MR (probably also WR), about 1 Ga old, is assumed to represent a metavolcanic sequence of an active continental margin. The juvenile VC rocks of the same age were derived from the same geotectonic environment. All three rock units collided in the lower crustal realm under amphibolite to granulite facies conditions. They were piled up as nappes (Fig. 9). Towards the end of the formation of the F4 largescale folds, the emplacement of the nappe piles happened. The upper nappes were transported over the lower nappes from N to S (see small arrows in Fig. 9). Indications for a younger EW compression perpendicular to the transport direction of the nappe pile were found. The F4 large-scale folds were curved from the N-S direction ( M a h i y a n g a n a ) t o NNW-SSE (Uraniya), NW-SE (Wellawaya) and finally to NE-SW (Kataragama) in the course of a Fs folding phase (see black arrows in Fig. 9 ). The HC rocks can be regarded as part of West-Gondwana, the MR, WR and VC rocks as part of East-Gondwana (Kr6ner, 1991 a). During the Pan-African event, 550 Ma ago, both units were welded together to form a single continent. This collision marks the final

amalgamation of East- and West-Gondwana into a supercontinent. In a Gondwana reconstruction Sri Lanka is juxtaposed either with the coast of Liitzow-HoIm Bay, East Antarctica (Lawver and Scotese, 1987; Yoshida et al., 1992; and other authors), or with the southeast coast of Madagascar (Kr6ner, 1991 a ). Just near the beginning of Cretaceous time (130 Ma) the Indian Shield, including Sri Lankaand Madagascar, were dispersed from the East Antarctic Shield (Johnson et al., 1976). Finally, the complex Cretaceous and Cenozoic spreading activities lead to the recent configuration.

Acknowledgements Support from the Deutsche Forschungsgemeinschaft ( D F G ) are gratefully acknowledged. The author also thanks U. Held, B. Miiller, J. Mezger and V. Hentschke for their help with field measurements and evaluation, his colleagues D. Czerwek and P. Smilde for their help with the statistical analysis and modelling and W. Jacoby, R. Meissner and W. G6tze for critical reading.

References Biichel, G., Czerwek, D., Held, U,, Hentschke, V., Mezger, J., Miiller, B., Smilde. P. and Vitange, P. W, in prep. Gravimetric investigations at the boundary be-

GRAVITY,MAGNETICAND STRUCTURALPATTERN,SRI LANKA

91

tween the Highland and Vijayan Complex, two lower crustal blocks in Sri Lanka. Cooray, P.G., 1969. The significance of mica ages from the crystalline rocks of Ceylon. Geol. Assoc. Can. Spec. Pap., 5: 47-57. Cooray, P.G., 1984. An Introduction to the Geology of Sri Lanka (Ceylon). National Museum of Sri Lanka Publication, Colombo, 2nd ed., 340 pp. Cooray, P.G., 1994. The Precambrian ofSri Lanka: a historical review. In: M. Raith and S. Hoernes (Editors), Tectonic, Metamorphic and Isotopic Evolution of Deep Crustal Rocks, With Special Emphasis on Sri Lanka. Precambrian Res., 66: 3-18 (this volume). Curray, J.R., 1991. Possible greenschist metamorphism at the base of a 22 km sedimentary section, Bay of Bengal. Geol., 19:1097-1100. Curray, J.R. and Moore, D.G., 1971. Growth of the Bengal Deep-Sea Fan and denudation in the Himalayas. Geol. Soc. Am. Bull., 82: 563-572. Curray, J.R., Emmel, F.J., Moore, D.G. and Raitt, R.W., 1982. Structure, tectonics and geological history of the northeastern Indian Ocean. In: A.E.M. Nairn and F.G. Stehli (Editors), The Ocean Basins and Margins, Vol. 6. The Indian Ocean. Plenum, New York, N.Y., pp. 399-450. Geological Survey Department of Sri Lanka, 1982. Geological Map of Sri Lanka. Scale: 8 miles to one inch, Colombo. Hatherton, T., Pattiaratchi, D.B. and Ranasinghe, V.V.C., 1975. Gravity map of Sri Lanka 1:1,000,000. Geol. Surv. Dept., Prof. Pap., 3, 39 pp. Johnson, B.D., Powell, C.McA. and Veevers, J.J., 1976. Spreading history of the eastern Indian Ocean and Greater India's northward flight from Antarctica and Australia. Geol. Soc. Am. Bull., 87:1560-1566. Kahle, H.-G., Naini, B.R., Talwani, M. and Eldholm, O., 1981. Marine geophysical study of the Comorin Ridge, North Central Indian Basin. J. Geophys. Res., 86 (B 5 ): 3807-3814. Kleinschrodt, R., 1994. Large-scale thrusting in the lower crustal basement of Sri Lanka. In: M. Raith and S. Hoernes (Editors), Tectonic, Metamorphic and Isotopic Evolution of Deep Crustal Rocks, With Special Emphasis on Sri Lanka. Precambrian Res., 66:39-57 (this volume). Kleinschrodt, R. and Voll, G., 1991. The eastern boundary of the central granulite belt. Excursion 5. In: G. Voll and R. Kleinschrodt (Editors), The Crystalline Crust of Sri Lanka, Part II. Excursion Guide. Geol. Surv. Dept. Sri Lanka, Prof. Pap., 6: 49-57. Kr~Sner, A., 1986. Composition, structure and evolution of the early Precambrian lower continental crust: constraints from geological observations and age relationships. Am. Geophys. Union, Geodyn. Ser., 14: 107119. Kr6ner, A., 1991a. African linkage of Precambrian Sri Lanka. Geol. Rundsch., 80: 429-440.

Kr6ner, A. (Editor), 199 lb. The Crystalline Crust of Sri Lanka, Part I. Summary of Research of German-Sri Lankan Consortium. Geol. Surv. Dept. Sri Lanka, Prof. Pap., 5,271 pp. Kr~Sner, A., Cooray, P.G. and Vitanage, P.W., 1991. Lithotectonic subdivision of the precambrian basement in Sri Lanka. In: A. Kr6ner (Editor), The Crystalline Crust of Sri Lanka, Part I. Summary of Research of German-Sri Lankan Consortium. Geol. Surv. Dept. Sri Lanka, Prof. Pap., 5: 5-21. Lawver, L.A. and Scotese, C.R., 1987. A revised reconstruction of Gondwanaland. In: D. McKenzie (Editor), Gondwana Six: Structure, Tectonics and Geophysics. Geophys. Monogr. Ser., 40:17-23. Liew, T.C, Milisenda, C.C. and Hofmann, A.W., 1991. Isotopic characterization of the high-grade basement rocks in Sri Lanka. In: A. KriSner (Editor), The Crystalline Crust of Sri Lanka, Part I. Summary of Research of German-Sri Lankan Consortium. Geol. Surv. Dept. Sri Lanka, Prof. Pap., 5: 258-267. Liu, C.-S., Sandwell, D.T and Curray, J.R., 1982. The negative gravity field over the 85°E Ridge. J. Geophys. Res., 87(B9): 7673-7686. Milisenda, C., 1991. Compositional characteristics of the Vijayan Complex. In: A. Kr6ner (Editor), The Crystalline Crust of Sri Lanka, Part I. Summary of Research of German-Sri Lankan Consortium. Geol. Surv. Dept. Sri Lanka, Prof. Pap., 5: 135-140. Milisenda, C., Liew, T.C., Hofmann, A.W. and Kr6ner, A., 1988. Isotopic mapping of age provinces in Precambrian high-grade terrains: Sri Lanka. J. Geol., 96: 608-615. Vitanage, P.W., 1985. Tectonics and mineralization in Sri Lanka. Geol. Soc. Finl. Bull., 57:157-168. Voll, G., 1991. The S-Highlands: the original state, S of the big synforms. Flat-lying metagranite and metabasite sills; metasediments, very open syn- and antiforms (F4), steep zones. In: G. Voll and R. Kleinschrodt (Editors), The Crystalline Crust of Sri Lanka, Part II. Excursion Guide. Geol. Surv. Dept. Sri Lanka, Prof. Pap., 6: 24-37. Voll, G. and Kleinschrodt, R. (Editors), 1991a. The Crystalline Crust of Sri Lanka, Part II. Excursion Guide. Geol. Surv. Dept. Sri Lanka, Prof. Pap., 6, 59 PP. Voll, G. and Kleinschrodt, R., 1991b. Sri Lanka: structural, magmatic and metamorphic development of a Gondwana fragment. In: A. Kr6ner (Editor), The Crystalline Crust of Sri Lanka, Part 1. Summary of Research of German-Sri Lankan Consortium. Geol. Surv. Dept. Sri Lanka, Prof. Pap., 5:22-51. Yoshida, M., Funaki, M. and Vitanage, P.W., 1992. Proterozoic to Mesozoic East Gondwana: the juxtaposition of India, Sri Lanka, and Antarctica. Tectonics, 11 (2): 381-391.